The purpose of this project is to continue to investigate immunity and other mechanisms of resistance to infection with Haemophilus influenzae type b. We are investigating the role of serum bactericidal activity in protection against Haemophilus influenzae type b disease by correlating in vitro data with protection in an experimental animal model of H. influenzae type b bacteremia and meningitis. In an effort to find alternative means of immunization against H. influenzae type b disease, we are studying possible protective effects of antibodies to noncapsular antigens of H. influenzae type b. We are also examining the genetics of the immune response and of susceptibility to infection with H. influenzae type b in inbred strains of rats.